Apparatus for directional solidification of silicon including a refractory material

ABSTRACT

The present invention relates to an apparatus and method for purifying silicon using directional solidification. The apparatus can be used more than once for the directional solidification of silicon without failure. The apparatus and method of the present invention can be used to make silicon crystals for use in solar cells.

PRIORITY CLAIM

This application claims the benefit of priority of U.S. application Ser.No. 12/947,936, filed Nov. 17, 2010, which is incorporated herein byreference in its entirety.

BACKGROUND

Solar cells can be a viable energy source by utilizing their ability toconvert sunlight to electrical energy. Silicon is a semiconductormaterial used in the manufacture of solar cells; however, a limitationof silicon use relates to the cost of purifying it to solar grade (SG).

Several techniques used to make silicon crystals for solar cells areknown. Most of these techniques operate on the principle that whilesilicon is solidifying from a molten solution, undesirable impuritiestend to remain in the molten solution. For example, the float zonetechnique can be used to make monocrystalline ingots, and uses a movingliquid zone in a solid material, moving impurities to edges of thematerial. In another example, the Czochralski technique can be used tomake monocrystalline ingots, and uses a seed crystal that is slowlypulled out of a solution, allowing the formation of a monocrystallinecolumn of silicon while leaving impurities in the solution. In yetanother example, the Bridgeman or heat exchanger techniques can be usedto make multicrystalline ingots, and use a temperature gradient to causedirectional solidification.

Various techniques for making silicon crystals for solar cells utilize acrucible to hold silicon during the molten manufacturing stage.Unfortunately, most crucibles break after a single use due to, forexample, the changing size or shape of the molten silicon as itsolidifies. Methods of generating monocrystalline ingots can include theuse of a quartz crucible, which is a costly and brittle material.Methods of generating multicrystalline ingots generally use a largercrucible, and due to the expense of quartz, these crucibles are oftenmade of cheaper materials such as fused silica or other refractorymaterials. Despite being made of cheaper materials, large crucibles madeof fused silica or other refractories are still costly to produce, andcan generally only be used once. The combination of high expense andlimited life of crucibles limits the economic efficiency of siliconpurification apparatus and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.Like numerals having different letter suffixes represent differentinstances of substantially similar components. The drawings illustrategenerally, by way of example, but not by way of limitation, variousembodiments discussed in the present document.

FIG. 1 illustrates a cross-sectional view of a mold, an outer jacket andan insulating layer of an apparatus for directional solidification ofsilicon, as constructed in accordance with at least one embodiment.

FIG. 2 illustrates a cross-sectional view of a mold, an outer jacket andan insulating layer of an apparatus for directional solidification ofsilicon, as constructed in accordance with at least one embodiment.

FIG. 3 illustrates a cross-sectional view of a heater of an apparatusfor directional solidification of silicon, as constructed in accordancewith at least one embodiment.

FIG. 4 illustrates a 3D projection of an apparatus for directionalsolidification of silicon, including a heater positioned on top of amold, as constructed in accordance with at least one embodiment.

FIG. 5 illustrates an isometric view of a heater of an apparatus fordirectional solidification of silicon, as constructed in accordance withat least one embodiment.

FIG. 6 illustrates an isometric view of a mold of an apparatus fordirectional solidification of silicon, as constructed in accordance withat least one embodiment.

FIG. 7 illustrates a silicon ingot generated by an apparatus and methodof the present invention, as constructed in accordance with at least oneembodiment.

SUMMARY

In view of current energy demands and supply limitations, the presentinventors have recognized a need for a more cost efficient way ofpurifying metallurgical grade (MG) silicone (or any other silicon havinga greater amount of impurities than solar grade) to solar grade silicon.

The present invention relates to an apparatus for directionalsolidification of silicon. The invention provides, among other things, adirectional solidification mold. The directional solidification mold caninclude at least one refractory material. The at least one refractorymaterial can be configured to allow directional solidification ofsilicon within the mold. An outer jacket and an insulating layer cansurround portions of the mold. In various examples, the insulating layeris disposed at least partially between the directional solidificationmold and the outer jacket. The present inventors have found that such amold configuration can be repeatedly used for the directionalsolidification of silicon without failure.

In some examples, one or more side walls of the directionalsolidification mold can include aluminum oxide. In some examples, abottom of the directional solidification mold can include siliconcarbide. The directional solidification mold can also include a toplayer. The top layer can include a slip-plane refractory. The top layercan be configured to sufficiently protect portions of the directionalsolidification mold from damage when a directionally solidified siliconproduct is removed. In some examples, the outer jacket of the mold caninclude steel or stainless steel. In some examples, the insulating layercan include an insulating brick, a refractory material, a mixture ofrefractory materials, an insulating board, a ceramic paper, a hightemperature wool, or combinations thereof.

In some examples, the invention also includes a heater. The heater caninclude one or more heating members. The one or more heating members canindependently include either a heating element or an induction heater. Aheating member that includes a heating element can utilize materialssuch as silicon carbide, molybdenum disilicide, graphite, orcombinations thereof. The heater can also include an insulating brick, arefractory, a mixture of refractories, an insulating board, a ceramicpaper, a high temperature wool, or combinations thereof. An outer jacketor insulation can surround portions of the heater. The outer jacket ofthe heater can include steel or stainless steel. The insulation of theheater can be at least partially disposed between the one or moreheating members and the outer jacket.

The present invention also relates to a method of purifying silicon. Themethod of purifying silicon can be a method for making one or moreingots for manufacture into solar cells. The method of purifying siliconcan be a method of making one or more silicon ingots for cutting intoone or more solar wafers. The method can include providing or receivinga first silicon and at least partially melting the first silicon. Themethod can include providing or receiving a directional solidificationapparatus. The silicon can be at least partially melted to provide afirst molten silicon. The method can further include directionallysolidifying the first molten silicon. The first silicon can bedirectionally solidified using the directional solidification apparatus.The directional solidification of the first molten silicon in thedirectional solidification apparatus can provide a second silicon. Theapparatus for directional silicon can include a directionalsolidification mold. The directional solidification mold can include atleast one refractory material. The directional solidification of thesilicon can occur in the directional solidification mold of thedirectional solidification apparatus. The refractory material can beconfigured to allow the directional solidification of silicon within themold. An outer jacket and an insulating layer can surround portions ofthe mold. The insulating layer is disposed at least partially betweenthe directional solidification mold and the outer jacket. Also, theapparatus can be repeatedly used for the directional solidification ofsilicon without failure.

In one specific embodiment, the present invention also relates to anapparatus for the purification of silicon. The apparatus includes adirectional solidification mold. The directional solidification moldincludes at least one refractory material. The at least one refractorymaterial is configured to allow the directional solidification ofsilicon within the mold. One or more sides of the directionalsolidification mold include aluminum oxide. A bottom of the directionalsolidification mold includes silicon carbide or graphite. Thedirectional solidification mold also includes a top layer. The top layerincludes a slip-plane refractory. The top layer is configured to protectthe remainder of the directional solidification mold from damage whendirectionally solidified silicon is removed from the mold. The inventionalso includes an outer jacket. The outer jacket includes steel orstainless steel. The invention also includes an insulating layer. Theinsulating layer is disposed at least partially between one or moresides of the directional solidification mold and one or more sides ofthe outer jacket. The insulating layer includes insulating brick, arefractory material, a mixture of refractory materials, insulatingboard, ceramic paper, high temperature wool, or a combination thereof.The specific embodiment also includes a top heater. The top heaterincludes one or more heating members. The one or more heating membersindependently include either a heating element or an induction heater.The heating element includes silicon carbide, molybdenum disilicide,graphite, or a combination thereof. The top heater also includesinsulating brick, a refractory, a mixture of refractories, insulatingboard, ceramic paper, high temperature wool, or a combination thereof.The top heater also includes an outer jacket. The outer jacket of thetop heater includes steel or stainless steel. The insulation of the topheater is at least partially disposed between the one or more heatingmembers and the top heater outer jacket. Also, the directionalsolidification mold, the outer jacket, and the insulating layer areconfigured to be used more than twice for the directional solidificationof silicon.

The present invention provides advantages over previous apparatus andmethods for directional solidification of silicon. In one example, thepresent invention can provide an economically more efficient method ofpurifying silicon due to reusability of the apparatus. For example, thepresent invention can provide an economically more efficient method ofmaking one or more silicon ingots for cutting into one or more solarwafers. The reusability of the apparatus can help to reduce waste, andcan provide a more economical way to use larger crucibles fordirectional solidification. By allowing the efficient use of largercrucibles, the present invention can enable directional solidificationmethods that benefit from the economics of scaling. Additionally, theheater present in some embodiments of the present invention offers aconvenient and efficient way to heat the silicon, maintain thetemperature of the silicon, control the cooling of the silicon, or acombination thereof, which can allow precise control over thetemperature gradient and the corresponding directional solidification ofthe silicon. The apparatus and method of the present invention can beused to make silicon crystals for use in solar cells, among otherthings.

DETAILED DESCRIPTION

Reference will now be made in detail to certain examples of thedisclosed subject matter, some of which are illustrated in theaccompanying drawings. While the disclosed subject matter will largelybe described in conjunction with the accompanying drawings, it should beunderstood that such descriptions are not intended to limit thedisclosed subject matter to those drawings. On the contrary, thedisclosed subject matter is intended to cover all alternatives,modifications, and equivalents, which can be included within the scopeof the presently disclosed subject matter, as defined by the claims.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

In this document, the terms “a” or “an” are used to include one or morethan one and the term “or” is used to refer to a nonexclusive “or”unless otherwise indicated. In addition, it is to be understood that thephraseology or terminology employed herein, and not otherwise defined,is for the purpose of description only and not of limitation.Furthermore, all publications, patents, and patent documents referred toin this document are incorporated by reference herein in their entirety,as though individually incorporated by reference. In the event ofinconsistent usages between this document and those documents soincorporated by reference, the usage in the incorporated referenceshould be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

In the methods of manufacturing described herein, the steps can becarried out in any order without departing from the principles of theinvention, except when a temporal or operational sequence is explicitlyrecited. Recitation in a claim to the effect that first a step isperformed, then several other steps are subsequently performed, shall betaken to mean that the first step is performed before any of the othersteps, but the other steps can be performed in any suitable sequence,unless a sequence is further recited within the other steps. Forexample, claim elements that recite “Step A, Step B, Step C, Step D, andStep E” shall be construed to mean step A is carried out first, step Eis carried out last, and steps B, C, and D can be carried out in anysequence between steps A and E, and that the sequence still falls withinthe literal scope of the claimed process. A given step or sub-set ofsteps can also be repeated.

Furthermore, specified steps can be carried out concurrently unlessexplicit claim language recites that they be carried out separately. Forexample, a claimed step of doing X and a claimed step of doing Y can beconducted simultaneously within a single operation, and the resultingprocess will fall within the literal scope of the claimed process.

The present invention relates to an apparatus and method for purifyingsilicon using directional solidification. The apparatus canadvantageously be used more than once for the directional solidificationof silicon. The apparatus and method of the present invention can beused to make silicon crystals for use in solar cells.

By controlling the temperature gradient in embodiments of the presentinvention, a highly controlled directional solidification can beaccomplished. In the present invention, the directional crystallizationproceeds approximately from bottom to top, thus the desired temperaturegradient has a lower temperature at the bottom and a higher temperatureat the top. High degrees of control over the temperature gradient andthe corresponding directional crystallization can advantageously allow amore effective directional solidification, resulting in a silicon ofhigher purity.

DEFINITIONS

As used herein, “conduit” refers to tube-shaped hole through a material,where the material is not necessarily tube-shaped. For example, a holerunning through a block of material is a conduit. The hole can be ofgreater length than diameter. A conduit can be formed by encasing a tube(including a pipe) in a material.

As used herein, “directional solidification” refers to crystallizing amaterial starting in approximately one location, proceeding in anapproximately linear direction (e.g. vertically, horizontally, orperpendicular to a surface), and ending in approximately anotherlocation. As used in this definition, a location can be a point, aplane, or a curved plane, including a ring or bowl shape.

As used herein, “fan” refers to any device or apparatus which can moveair.

As used herein, “heating element” refers to a piece of material whichgenerates heat when electricity is allowed to flow through thatmaterial.

As used herein, “induction heater” refers to a heater which adds heat toa material via the inducement of electrical currents in that material.Generally, such electrical currents are generated by allowing analternating current to travel through a coil of metal that is proximateto the material to be heated.

As used herein, “melt” refers to undergoing the phase transition fromsolid to liquid.

As used herein, “oil” refers to a substance that is liquid at ambienttemperature, that is hydrophobic, and that has a boiling point above300° C. Examples of oils include but are not limited to vegetable oilsand petroleum oils.

As used herein, “refractory material” refers to a material which ischemically and physically stable at high temperatures. Examples ofrefractory materials include but are not limited to aluminum oxide,silicon oxide, magnesium oxide, calcium oxide, zirconium oxide, chromiumoxide, silicon carbide, graphite, or a combination thereof.

As used herein, “hot face refractory” refers to a refractory material.

As used herein, “conducting refractory” refers to a refractory materialthat can conduct heat.

As used herein, “side” or “sides” can refer to one or more sides, andunless otherwise indicated refers to the side or sides or an object ascontrasted with one or more tops or bottoms of the object.

As used herein, “silicon” refers to the element Si, and can refer to Siin any degree of purity, but generally refers to silicon that is atleast 50% by weight pure, preferably 75% by weight pure, more preferably85% pure, more preferably 90% by weight pure, and more preferably 95% byweight pure, and even more preferably 99% by weight pure.

As used herein, “slip-plane refractory” refers to a refractory materialthat decreases friction and decreases sticking between the solid siliconand the directional solidification mold.

As used herein, “tube” refers to a hollow pipe-shaped material. A tubegenerally has an internal shape that approximately matches its outershape. The internal shape of a tube can be any suitable shape, includinground, square, or a shape with any number of sides, includingnon-symmetrical shapes.

Bottom Mold

FIG. 1 illustrates an embodiment of the present invention. A side-viewcut-away of the apparatus 100 is shown. Apparatus 100 includes adirectional solidification mold 110, which includes at least onerefractory material. The at least one refractory material is configuredto allow directional solidification of silicon within the mold. Theapparatus 100 also includes an outer jacket 130. Additionally, theapparatus 100 includes an insulating layer 120, disposed at leastpartially between the directional solidification mold 110 and the outerjacket 130. The apparatus 100 can be used more than once for thedirectional solidification of silicon.

The overall three-dimensional shape of an embodiment of the inventioncan be similar to a thick-walled large bowl, having a circular shape.Alternatively, the overall shape can be similar to a large bowl, havinga square shape, or a hexagon, octagon, pentagon, or any suitable shape,with any suitable number of edges. In other embodiments, the overallshape of the apparatus can be any suitable shape for directionalsolidification of silicon. In one embodiment, the bottom mold can holdabout 1 metric tonne of silicon, or more. In one embodiment, the bottommold can hold about 1.4 metric tonnes of silicon, or more. In anotherembodiment, the bottom mold can hold about 2.1 metric tonnes of silicon,or more. In another embodiment, the bottom mold can hold approximately1.2, 1.6, 1.8, 2.0, 2.5, 3, 3.5, 4, 4.5, or 5 metric tonnes of silicon,or more.

In preferred embodiments of the present invention, the apparatus isapproximately symmetrical about a center vertical axis. An embodiment inwhich the materials included in the apparatus or the shape of theapparatus deviate from close symmetry about a center axis is stillincluded as a preferred embodiment; the preference for symmetry isapproximate, as will readily be understood by one of skill in the art.In some embodiments, the apparatus is not symmetrical about a centervertical axis. In other embodiments, the apparatus is partiallyapproximately symmetrical about a center vertical axis and partiallynon-symmetrical about a center vertical axis. In embodiments thatinclude non-symmetric features, any suitable feature can be includedthat is described herein, including features described as being part ofan embodiment that is approximately symmetric about a center axis inwhole or in part.

As shown in FIG. 1, the directional solidification mold 110 has agreater than 90 degree internal angle between the bottom of thedirectional solidification mold and the sides of the directionalsolidification mold, referred to as a draft herein. The draft allows apiece of silicon solidified in the mold to be removed without having tobreak the silicon or the directional solidification mold. In a preferredembodiment, the directional solidification mold has a draft, as shown inFIG. 1, sufficient to allow the removal of silicon from the mold asdescribed. However, in alternative embodiments, the directionalsolidification has no draft, or has the reverse of a draft. Inalternative embodiments with no draft, the apparatus preferably willhave a cut through the middle that allows it to be easily parted intotwo halves for removal of the solid silicon. The two halves can then berejoined to again form a whole, and the apparatus reused. However,embodiments that can be broken into two halves are not restricted tothose embodiments that lack draft. All embodiments discussed herein caneither include or not include the ability to be separated into halvesfor easy removal of the solid silicon.

The directional solidification mold 110 shown in FIG. 1 includes arefractory material. The refractory material can be any suitablerefractory material. The refractory material can be aluminum oxide,silicon oxide, magnesium oxide, calcium oxide, zirconium oxide, chromiumoxide, silicon carbide, graphite, or a combination thereof. Thedirectional solidification mold can include one refractory material. Thedirectional solidification mold can include more than one refractorymaterial. The refractory material or materials that are included in thedirectional solidification mold can be mixed, or they can be located inseparate parts of the directional solidification mold, or a combinationthereof. The one or more refractory materials can be arranged in layers.The directional solidification mold can include more than one layer ofone or more refractory materials. The directional solidification moldcan include one layer of one or more refractory materials. The sides ofthe refractory can be a different material than the bottom of therefractory. The sides of the directional solidification mold as comparedto the bottom of the directional solidification mold can be differentthicknesses, include different compositions of material, includedifferent amounts of material, or a combination thereof. In oneembodiment, the sides of the directional solidification mold include ahot face refractory, and the bottom of the directional solidificationmold includes a conducting refractory. The sides of the directionalsolidification mold can include aluminum oxide. The bottom of thedirectional solidification mold can include a heat-conductive material,such as, for example, silicon carbide, graphite, steel, stainless steel,cast iron, copper, or a combination thereof.

In some embodiments, the material or materials that are included in thesides of the directional solidification mold can extend from a height ofthe external-bottom of the directional solidification mold and upwards,and the material or materials that are included in the bottom of thedirectional solidification mold can extend vertically from a verticalposition corresponding to the inside of one side of the directionalsolidification mold across the bottom to the a vertical positioncorresponding to the inside of the opposite side. In another embodiment,the material or materials that are included in the sides of thedirectional solidification mold can extend from a height of theinternal-bottom of the directional solidification mold and upwards,while the material or materials that are included in the bottom of thedirectional solidification mold can extend vertically from a verticalposition corresponding to the outside of one side of the directionalsolidification mold across the bottom of the directional solidificationmold to a vertical position corresponding to the outside of the oppositeside of the directional solidification mold. In another example, thematerial or materials that are included in the sides of the directionalsolidification mold can extend from a height above the height of thebottom of the directional solidification mold and upwards, while thematerial or materials that are included in the bottom of the directionalsolidification mold can extend vertically across the bottom of thedirectional solidification mold from a vertical position correspondingto one external side of the directional solidification mold to avertical position corresponding to the other external side of thedirectional solidification mold, and also extend up the sides above theheight of the bottom of the directional solidification mold. In anotherexample, the material or materials that are included in the sides of thedirectional solidification mold can extend from a height of theinternal-bottom of the directional solidification mold and upwards,while the material or materials that are included in the bottom of thedirectional solidification mold can extend vertically from the innerside of a side of the outer jacket across the inner-bottom of the outerjacket to the inner side of the opposite side of the outer jacket, orthe material or material or materials that are included in the bottom ofthe directional solidification mold can extend vertically fromin-between the inner side of a side of the outer jacket and the verticalposition corresponding to the outer side of the directionalsolidification mold, across the bottom of the directional solidificationmold, to in-between the opposite inner side of the outer jacket and thevertical position corresponding to the outer side of the opposite sideof the directional solidification mold.

The insulating layer 120 of apparatus 100 shown in FIG. 1 can include aninsulating material. The insulating material can be any suitablematerial. For example, the insulating material can be insulating brick,a refractory material, a mixture of refractory materials, insulatingboard, ceramic paper, high temperature wool, or a combination thereof.Insulating board can include high temperature ceramic board. Theinsulating layer can include more than one insulating material. Theinsulating material or materials that are included in the insulatinglayer 120 can be blended, mixed, or they can be located in separateparts of the insulating layer, or a combination thereof. The one or moreinsulating materials can be arranged in layers. In one example, theinsulating layer can include more than one layer of one or moreinsulating materials. In another example, the insulating layer caninclude one layer of one or more insulating materials. The sides of theinsulating layer can be a different material than the bottom of theinsulating layer. For example, the sides of the insulating layer ascompared to the bottom of the insulating layer can be differentthicknesses, include different compositions of material, or acombination thereof. In one embodiment, the insulating layer is disposedbetween the bottom of the directional solidification mold and the outerjacket. In a preferred embodiment, the insulating layer is disposed atleast partially between the sides of the directional solidification moldand the sides of the outer jacket, and the insulating layer is notdisposed between the bottom of the directional solidification mold andthe bottom of the outer jacket, as depicted in FIG. 1.

The insulating layer 120 of apparatus 100 is shown in FIG. 1 disposedbetween the sides of the outer jacket of the apparatus and the sides ofthe directional solidification mold of the apparatus. As shown, thesides of the insulating layer extend from a height corresponding to theinner side of the bottom of the outer jacket and upwards. Embodiments ofthe present invention also can include configurations of the insulatinglayer 120 in which the insulating layer extends from a heightcorresponding to the bottom of the inside of the directionalsolidification mold and upwards. In another example, the insulatinglayer extends from in-between the inner side of the bottom of the outerjacket and the height of the inner side of the bottom of the outerjacket and upwards. In another embodiment, the insulating layer extendsfrom above the height corresponding to the bottom of the inside of thedirectional solidification mold and upwards.

The outer jacket 130 of apparatus 100 shown in FIG. 1 can include anysuitable material to enclose the insulating layer and the directionalsolidification mold. The outer jacket can include one or more materials.In one embodiment, the outer jacket includes steel. In anotherembodiment, the outer jacket includes steel, stainless steel, copper,cast iron, a refractory material, a mixture of refractory materials, ora combination thereof. Different portions of the outer jacket caninclude different materials, different thicknesses of materials,different compositions of materials, or a combination thereof.

In some embodiments, the outer jacket can include structural members.The structural members can add strength and rigidity to the apparatusand can include any suitable material. For example, the structuralmembers can include steel, stainless steel, copper, cast iron, arefractory material, a mixture of refractory materials, or a combinationthereof. In one example, the outer jacket can include one or morestructural members that extend from the outside of the outer jacket in adirection that is away from the center of the apparatus, and that extendhorizontally around the circumference or perimeter of the apparatus. Theone or more horizontal structural members can be located, for example,at the upper edge of the outside of the outer jacket, at the bottom edgeof the outside of the outer jacket, at any position in-between the topand bottom edges of the outside of the outer jacket. In one example, theapparatus includes three horizontal structural members, with one locatedat the upper edge of the outer jacket, one located at the bottom edge ofthe outer jacket, and one located in-between the upper and lower edgesof the outer jacket. The outer jacket can include one or more structuralmembers on the outside of the outer jacket that extend from the outsideof the of the outer jacket in a direction that is away from the centerof the apparatus, and that extend vertically from the bottom of theoutside of the outer jacket to the top of the outside of the outerjacket. In one example, the outer jacket can include eight verticalstructural members. The vertical structural members can be evenly spacedaround the circumference or perimeter of the outer jacket. In anotherexample, the outer jacket includes both vertical and horizontalstructural members. The outer jacket can include structural members thatextend across the bottom of the outer jacket. The structural member onthe bottom can extend from one outer edge of the bottom of the outerjacket to another edge of the bottom of the outer jacket. A structuralmembers on the bottom can also extend partially across the bottom of theouter jacket. Structural members can be strips, bars, tubes, or anysuitable structure for adding structural support to the apparatus. Astructural member can be attached to the outer jacket via welding,brazing, or any other suitable method. The structural members can beadapted to facilitate transportation and physical manipulation of theapparatus. For example, the structural members on the bottom of theoutside of the outer jacket can be tubes of sufficient size, strength,orientation, spacing, or a combination thereof, such that a particularfork-lift or other lifting machine could lift or move or otherwisephysically manipulate the apparatus. In another embodiment, thestructural members described above as being located on the outside ofthe outer jacket can alternatively or additionally be located on theinside of the outer jacket.

The outer jacket 130 is shown in FIG. 1 as extending over the top of theinsulating layer 120 and partially covering the top of the directionalsolidification mold 110. However, embodiments of the present inventionalso encompass a wide range of various structural configurations of theouter jacket 130 with respect to the insulating layer 120 and thedirectional solidification mold 110. Embodiments can include an outerjacket 130 extending completely to the inner rim of the top of thedirectional solidification mold 110, the outer jacket 130 extending onlypart way across the top of the insulating layer 120, or the outer jacket130 not extending across any part of the insulating layer 120. Alsoincluded are configurations in which the outer jacket 130 does notextend fully up the side of the outside of the insulating layer. In anembodiment in which the outer jacket extends over any portion of the topof the directional solidification mold or the insulating layer, theportion of the outer jacket that extends over the top can include amaterial with greater insulating properties than the sides and bottom ofthe outer jacket. In some embodiments, such a choice of material canencourage formation of a desired temperature gradient in the apparatus.

The top edge of the apparatus 100 shown in FIG. 1 is depicted with thedirectional solidification mold 110 and the insulating layer 120 at anapproximately even height, and with the top of the outer jacketextending over the top of the insulating layer and partially over thetop of the directional solidification layer. As discussed above, otherconfigurations of the top of the outer jacket, the top of the insulatinglayer, and the top of the directional solidification mold areencompassed as embodiments of the present invention, including allsuitable arrangements. For example, the insulating layer can extendvertically to a height above the top edge of the directionalsolidification mold. Alternatively, the directional solidification moldcan extend vertically to a height above the top edge of the directionalsolidification mold. The insulating layer can extend partially orcompletely over the top edge of the directional solidification mold. Or,the directional solidification mold can extend partially or completelyover the top edge of the insulating layer.

The apparatus 100 shown in FIG. 1 is depicted with particular relativethicknesses of the directional solidification mold 110, the insulatinglayer 120, and the outer jacket 130. However, embodiments of the presentinvention encompass any suitable relative thickness of the mold 110,insulation 120, and outer jacket 130.

The apparatus 100 in FIG. 1 can be used more than once to directionallysolidify silicon. In being able to be used more than once, the apparatuscan be reused for directional solidification with no repairs betweenuses, or with minimal repairs between uses. Minimal repairs can includetouching up or reapplying entirely a coating that is part of the innerside of the directional solidification mold, for example repairing a toplayer, including a slip-plane refractory coating. Embodiments of thepresent invention also include an apparatus that can be used more thantwice for directional solidification of silicon. Also included areapparatus that can be used more than three, four, five, six, twelve, ormore times, for the directional solidification of silicon.

In some embodiments of the present invention the apparatus only includesa bottom mold. In other embodiments, the present invention includes botha bottom mold and a top heater.

FIG. 2 illustrates an embodiment of the present invention. A side-viewcut-away of the apparatus 200 is shown. Apparatus 200 includes adirectional solidification mold 201, which includes at least onerefractory material. The at least one refractory material is configuredto allow directional solidification of silicon within the mold. Theapparatus 200 also includes an outer jacket 203. Additionally, theapparatus includes an insulating layer 202, disposed at least partiallybetween the directional solidification mold 201 and the outer jacket203. The apparatus 100 can be used more than once for the directionalsolidification of silicon.

The directional solidification mold 201 shown in FIG. 2 includes one ormore refractory materials. The sides of the directional solidificationmold include a hot face refractory 220. In FIG. 2, the hot facerefractory 220 of the directional solidification mold extends from theinside of the bottom of the outer jacket and upwards; as discussedabove, various configurations of the sides of the directionalsolidification mold are encompassed as embodiments of the presentinvention. The hot face refractory 220 can be any suitable refractorymaterial. For example, the hot face refractory 220 can be aluminumoxide.

In building the bottom mold apparatus of the present invention,refractory materials can be applied in a manner similar to applying wetcement. A trawl or other suitable implement, including forms, can beused to manipulate the wet refractory into the desired shape, followedby allowing the refractory material to dry and set.

The bottom of the directional solidification mold 201 shown in FIG. 2includes a conducting refractory 230. In FIG. 2, the conductingrefractory 230 of the directional solidification mold extends verticallyin-between a vertical position corresponding to the outside of a side ofthe directional solidification mold and a vertical positioncorresponding to the inside of a side of the directional solidificationmold, across the bottom of the directional solidification mold, toin-between the a vertical position corresponding to the outside of theopposite side of the directional solidification mold and a verticalposition corresponding to the outside of the opposite side of thedirectional solidification mold; as discussed above, variousconfigurations of the sides of the directional solidification mold areencompassed as embodiments of the present invention. The conductingrefractory 230 can be any suitable material. For example, the conductingrefractory can contain silicon carbide. By placing a conducting materialon the bottom of the apparatus, cooling of the bottom of molten siliconthat is within the directional solidification mold is facilitated.Facile cooling of the bottom of the mold assists in forming andcontrolling a temperature gradient between the bottom and the top of thedirectional solidification mold, allowing the desired directionalsolidification to take place within the mold beginning at the bottom andending at the top.

In an alternative embodiment, the bottom of the directionalsolidification mold can include any suitable heat-conducting materialfor element 230, including silicon carbide, graphite, copper, steel,stainless steel, graphite, cast iron, or a combination thereof. As withthe embodiment shown in FIG. 2, such an embodiment can include a toplayer 210. Alternatively, such an embodiment does not include a toplayer 210.

Conducting refractory 230 is shown in FIG. 2 with an appendage member231 at its outer edge. The appendage member 231 of the conductingrefractory 230 secures the conducting refractory into a receiving slot232 located in the hot face refractory 220. Securing the hot facerefractory to the conducting refractory prevents it from coming loosefrom the apparatus. In one embodiment, the appendage 231 and thereceiving slot 232 are included. In another embodiment, they are notincluded. In other embodiments, alternative means of securing theconducting refractory are included.

The directional solidification mold 201 shown in FIG. 2 also includes atop layer 210. The top layer includes at least one slip-plane refractorymaterial. The slip-plane refractory material can include any suitablerefractory material. The slip-plane refractory material includes fusedsilicon dioxide, silicon dioxide, aluminum oxide, silicon nitride,graphite, or a combination thereof. The top layer 210 is such that itprotects the remainder of the directional solidification mold fromdamage when directionally solidified silicon is removed from the mold.For example, the remainder of the directional solidification mold 201 inFIG. 2 is the hot face refractory 220 and the conducting refractory 230.The top layer 210 can be of approximately consistent thickness andcomposition throughout, as shown in FIG. 2. In other embodiments, thetop layer can have variable thickness or composition. Alternatively,some portions of the top layer can be of approximately consistentthickness and composition, and other portions of the top layer can havevariable thickness or composition. In protecting the remainder of thedirectional solidification mold from damage when the solid silicon isremoved, and in facilitating the removal of the solid silicon, the toplayer can become damaged in part or in full when the silicon is removed.The top layer can be replaced or repaired in-between one or more uses ofthe apparatus. The top layer can be applied in any suitable manner. Thetop layer can be applied as a spray, or brushed on. In another examplethe top layer can be applied using a trawl, and spread like wet cement.After application, the top layer can be allowed to dry and set. In someembodiments, colloidal silica can be used as a binder for the top layer,including for the slip plane refractory spray. The top layer can beheated before it is used to dry it out and prepare it for use.

The insulating layer 202 of apparatus 200 shown in FIG. 2 is disposed atleast partially between the sides of the directional solidification mold201 and the outer jacket 203. As shown in FIG. 2, the insulating layerof a specific embodiment extends from the inside of the bottom of theouter jacket and upwards. As discussed above, various configurations ofthe insulating layer are encompassed as embodiments of the presentinvention. The insulating layer 202 shown in FIG. 2 includes two layers,inner layer 240 and outer layer 250. The layers 240 and 250 can includeany suitable insulating material. In one embodiment, outer insulatinglayer 250 includes ceramic paper and high temperature ceramic board. Inone embodiment, outer insulating layer 250 includes ceramic paper, hightemperature wool, high temperature ceramic board, or a combinationthereof. In one embodiment, inner insulating layer 240 includesinsulating brick or a refractory material, where the refractory materialcan include a castable refractory material.

The outer jacket 203 of apparatus 200 shown in FIG. 2 includes anysuitable material. For example, outer jacket 203 includes steel orstainless steel. In FIG. 2, the outer jacket is shown extending over thetop of the outer layer 250 and partially over the top of the inner layer240; as discussed above, various configurations of the insulating layerare encompassed as embodiments of the present invention.

The apparatus 200 shown in FIG. 2 includes anchors 260. The anchors cansecure the refractory layers in the apparatus, to prevent them fromloosening. For example, in FIG. 2, the anchors 260 secure the hot facerefractory 220 to the inner layer 240, and can help to secure theapparatus. In other embodiments, the anchors can be fixed in the outerjacket and can extend through any number of layers to secure them. Inother embodiments, the anchors can start and end in any suitable layers.The anchors can include any suitable material. For example, the anchorscan include steel, stainless steel, or cast iron. The anchors can be anysuitable shape, and in any suitable orientation. By securing theapparatus with anchors 260, with the appendage 231 and slot 232, with acombination thereof, or with an alternative means of securing, theapparatus can have a longer lifetime and can withstand more variedtreatments with minimized damage. Additionally, securing the apparatuswith anchors 260, with the appendage 231 and slot 232, with acombination thereof, or with alternative securing means, can help toprevent the layers from falling out of the apparatus if the apparatus isinverted.

Top Heater

In one embodiment, the present invention also includes a top heater. Thetop heater can be positioned on top of the bottom mold. The shape of thebottom of the top heater approximately matches the shape of the top ofthe bottom mold. The top heater can apply heat to the top of the bottommold, heating the silicon therein. Application of heat to the bottommold can cause melting of silicon in the bottom mold. Additionally,application of heat to the bottom mold can allow control of thetemperature of the silicon in the bottom mold. Also, the top heater canbe positioned on top of the bottom mold without heating, serving as aninsulator to control the release of heat from the top of the bottommold. By controlling the temperature or release of heat of the top ofthe bottom mold, the desired temperature gradient can be more easilyaccomplished, which can allow a more highly controlled directionalsolidification. Ultimately, control over the temperature gradient canallow a more effective directional solidification in which the resultingpurity of the silicon is maximized. In one embodiment, type Bthermocouples can be used to monitor the temperature inside the furnacechamber.

FIG. 3 illustrates a top heater 300. The top heater can include one ormore heating members 310. Each of the one or more heating members canindependently include any suitable material. For example, each of theone or more heating members independently can include a heating element,where the heating element can include silicon carbide, molybdenumdisilicide, graphite, or a combination thereof; and, each of the one ormore heating members can alternatively independently include aninduction heater. In one embodiment, the one or more heating members arepositioned at approximately the same height. In another embodiment, theone or more heating members are positioned at different heights.

In one example, the heating elements include silicon carbide, which hascertain advantages. For example, silicon carbide heating elements do notcorrode at high temperatures in the presence of oxygen. Oxygen corrosioncan be reduced for heating elements including corrodible materials byusing a vacuum chamber, but silicon carbide heating elements can avoidcorrosion without a vacuum chamber. Additionally, silicon carbideheating elements can be used without water-cooled leads. In oneembodiment, the heating elements are used in a vacuum chamber, withwater-cooled leads, or both. In another embodiment, the heating elementsare used without a vacuum chamber, without water-cooled leads, orwithout both.

In one embodiment, the one or more heating members are inductionheaters. The induction heaters can be cast into one or more refractorymaterials. The refractory material containing the induction heating coilor coils can then be positioned over the bottom mold. The refractorymaterial can be any suitable material. For example, the refractorymaterial can include aluminum oxide, silicon oxide, magnesium oxide,calcium oxide, zirconium oxide, chromium oxide, silicon carbide,graphite, or a combination thereof. In another embodiment, the inductionheaters are not cast into one or more refractory materials.

In one embodiment the one or more heating members have an electricalsystem such that if at least one heating member fails, any remainingfunctional heating members continue to receive electricity and toproduce heat. In one embodiment, each heating member has its owncircuit.

The top heater can include insulation, for example top heater 300 shownin FIG. 3 includes insulation 320. The insulation can include anysuitable insulating material. The insulation can include one or moreinsulating materials. For example, the insulation can include insulatingbrick, a refractory, a mixture of refractories, insulating board,ceramic paper, high temperature wool, or a mixture thereof. Insulatingboard can include high temperature ceramic board. As shown in FIG. 3,the bottom edge of the insulating material and the one or more heatingmembers 310 are at approximately the same height. Other configurationsof the one or more heating members and the insulation are encompassed asembodiments of the present invention. For example, the one or moreheating members can include an induction heater, the insulation caninclude a refractory material, and the one or more heating members canbe encased in the refractory material. In such an embodiment, additionalinsulating material can also be optionally included, where theadditional insulation can be refractory material, or the additionalinsulation can be another suitable insulating material. In anotherexample, the one or more heating members can include an inductionheater, and the heating member can be positioned as the heating memberis shown in FIG. 3, or in another configuration similarly not encased inrefractory material. In another example, the one or more heating memberscan be positioned above the height of the bottom edge of the insulation.In another example, the bottom edge of the insulation can be positionedabove the height of the one or more heating members. In an embodiment inwhich the one or more heating members are positioned at differentheights, the edge of the insulation can be in-between the heights of theheating members, or in any other configuration as described above.

The top heater can include an outer jacket, for example top heater 300shown in FIG. 3 includes outer jacket 330. The outer jacket can includeany suitable material. For example, the outer jacket can include steelor stainless steel. In another embodiment, the outer jacket includessteel, stainless steel, copper, cast iron, a refractory material, amixture of refractory materials, or a combination thereof. Theinsulation 320 is disposed at least partially between the one or moreheating members and the outer jacket. In FIG. 3, the bottom edge of theouter jacket 330 is shown to be approximately even with the bottom edgeof the insulation and with the one or more heating members. However, asdiscussed with respect to the one or more heating members and theinsulation above, various configurations of the outer jacket, theinsulation, and the one or more heating members are encompassed asembodiments of the present invention. For example, the edge of outerjacket can extend below the edge of the insulation and the one or moreheating members. In another example, the edge of the outer jacket canextend below the edge of the insulation, below the one or more heatingmembers, or a combination thereof. In one example, the outer jacket canextend below the bottom edge of the insulation and continue acrosseither fully or partially covering the bottom edge of the insulation. Insome embodiments, the portion of the outer jacket that covers the edgeof the insulation can include a material with a relatively lowconductivity, such as a suitable refractory, such as aluminum oxide,silicon oxide, magnesium oxide, calcium oxide, zirconium oxide, chromiumoxide, silicon carbide, graphite, or a combination thereof. In anotherexample, the outer jacket does not extend below the bottom edge of theinsulation or the height of the one or more heating members. In anotherembodiment, the outer jacket extends below the height of the one or moreheating members, but is still above the bottom edge of the insulation.In an embodiment in which the one or more heating members are positionedat different heights, the outer jacket can extend to a height that isin-between the heights of the heating elements, or in any otherconfiguration as described above.

In some embodiments, the top heater outer jacket can include structuralmembers. The structural members can add strength and rigidity to the topheater. The structural members can include steel, stainless steel,copper, cast iron, a refractory material, a mixture of refractorymaterials, or a combination thereof. In one example, the top heaterouter jacket can include one or more structural members that extend fromoutside of the top heater outer jacket in a direction that is away fromthe center of the top heater, and that extend horizontally around thecircumference or perimeter of the top heater. The one or more horizontalstructural members can be located, for example, at the lower edge of theoutside of the top heater outer jacket, at the top edge of the outsideof the top heater outer jacket, at any position in-between the bottomand top edges of the outside of the top heater outer jacket. In oneexample, the top heater includes three horizontal structural members,with one located at the bottom edge of the top heater outer jacket, onelocated at the upper edge of the top heater outer jacket, and onelocated in-between the lower and upper edges of the top heater outerjacket. The top heater outer jacket can include one or more structuralmembers on the outside of the top heater outer jacket that extend foroutside of the top heater outer jacket in a direction that is away fromthe center of the top heat vertically from the bottom of the outside ofthe top heater outer jacket to the top of the outside of the top heaterouter jacket. In one example, the top heater outer jacket can includeeight vertical structural members. The vertical structural members canbe evenly spaced around the circumference or perimeter of the topheater. In another example, the top heater outer jacket includes bothvertical and horizontal structural members. The top heater outer jacketcan include structural members that extend across the top of the topheater outer jacket. The structural member on the top can extend fromone outer edge of the top of the top heater outer jacket to another edgeof the top of the top heater outer jacket. The structural members on thetop can also extend partially across the top of the outer jacket. Thestructural members can be strips, bars, tubes, or any suitable structurefor adding structural support to the top heater. The structural memberscan be attached to the top heater outer jacket via welding, brazing, orother suitable method. The structural members can be adapted tofacilitate transportation and physical manipulation of the apparatus.For example, the structural members on the top of the outside of the topheater outer jacket can be tubes of sufficient size, strength,orientation, spacing, or a combination thereof, such that a particularfork-lift or other lifting machine could lift or move or otherwisephysically manipulate the top heater. In another embodiment, thestructural members described above as being located on the outside ofthe top heater outer jacket can alternatively or additionally be locatedon the inside of the top heater outer jacket. In another embodiment, thetop heater can be moved using a crane or other lifting device, usingchains attached to the top heater, including chains attached tostructural members of the top heater or to non-structural members of thetop heater. For example, four chains could be attached to the upper edgeof the top heater outer jacket to form a bridle for a crane to lift andotherwise move the top heater.

Cooling

As discussed above, by controlling the temperature gradient in theapparatus, a highly controlled directional solidification can beaccomplished. High degrees of control over the temperature gradient andthe corresponding directional crystallization can allow a more effectivedirectional solidification, providing a silicon of high purity. In thepresent invention, the directional crystallization proceeds fromapproximately bottom to top, thus the desired temperature gradient has alower temperature at the bottom and a higher temperature at the top. Inembodiments with a top heater, the top heater is one way to control theentry or loss of heat from the top of the directional solidificationmold. Some embodiments include a conducting refractory material in thedirectional solidification mold to induce heat loss from the bottom ofthe apparatus, while some embodiments also include insulating materialon the sides of the directional solidification mold to prevent heat losstherefrom and to both encourage the formation of a vertical thermalgradient and to discourage the formation of a horizontal thermalgradient. In some methods of using the present invention, fans can beblown across the bottom of the apparatus, for example across the bottomof the outer jacket, to control heat loss from the bottom of theapparatus. In some methods of using the present invention, circulationof ambient air without the use of a fan is used to cool the apparatus,including the bottom of the apparatus.

In some embodiments of the present invention, one or more heat transferfins can be attached to the bottom of the outer jacket to facilitate aircooling of the apparatus. Fans can enhance the cooling effect of coolingfins by blowing across the bottom of the outer jacket. Any suitablenumber of fins can be used. The one or more fins can absorb heat fromthe bottom of the apparatus and allow the heat to be removed by aircooling, facilitated by the surface area of the fin. For example, thefins can be made of copper, cast iron, steel, or stainless steel.

In some embodiments of the present invention, there is included at leastone liquid conduit. The at least one liquid conduit is configured toallow a cooling liquid to pass through the conduit, thereby transferringheat away from the directional solidification mold. The cooling liquidcan be any suitable cooling liquid. The cooling liquid can be oneliquid. The cooling liquid can be a mixture of more than one liquid. Thecooling liquid can include water, ethylene glycol, diethylene glycol,propylene glycol, an oil, a mixture of oils, or a combination thereof.

In some embodiments, the at least one liquid conduit includes a tube.The tube can include any suitable material. For example, the tube caninclude copper, cast iron, steel, stainless steel, a refractorymaterial, a mixture of refractory materials, or a combination thereof.The at least one liquid conduit can include a conduit through amaterial. The conduit can be through any suitable material. For example,the conduit can be through a material that includes copper, siliconcarbide, graphite, cast iron, steel, stainless steel, a refractorymaterial, a mixture of refractory materials, or a combination thereof.The at least one liquid conduit can be a combination of a tube and aconduit through a material. In some embodiments, the at least one liquidconduit can be located adjacent to the bottom of the apparatus. The atleast one liquid conduit can be located within the bottom of theapparatus. The location of the at least one liquid conduit can include acombination of being adjacent to the bottom of the apparatus and beingwithin the bottom of the apparatus.

The liquid conduit included in some embodiments of the present inventionencompasses a variety of configurations that allow a cooling liquid totransfer heat away from the directional solidification mold. A pump canbe used to move the cooling liquid. A cooling system can be used toremove heat from the cooling liquid. For example, one or more tubes,including pipes, can be used. The one more tubes can be any suitableshape, including round, square, or flat. The tubes can be coiled. Thetubes can be adjacent to the outside of the outer jacket. In preferredembodiments, the tubes can be adjacent to the bottom of the outside ofthe outer jacket. The tubes can contact the outer jacket such thatsufficient surface area contact occurs to allow efficient transfer ofheat from the apparatus to the cooling liquid. The tubes can contact theouter jacket in any suitable fashion, including along an edge of a tube.The tubes can be welded, brazed, soldered, or attached by any suitablemethod to the outside of the outer jacket. The tubes can be flattened tothe outside of the outer jacket to enhance the efficiency of heattransfer. In some embodiments, the at least one liquid conduit is one ormore conduits running through the bottom of the bottom mold. A conduitrunning through the bottom of the bottom mold can be a tube encased in arefractory that is included in the directional solidification mold. Atube can enter one part of the outer jacket, run through a refractorymaterial or conductive material or a combination thereof at the bottomof the directional solidification mold, and exit another part of theouter jacket. A tube encased in the bottom refractory or bottomconductive material of the directional solidification mold can becoiled, or arranged in any suitable shape, including moving back andforth one or more times before exiting the bottom of the apparatus.

In another embodiment, the at least one liquid conduit includes a tubeencased in a refractory material, a heat-conductive material, or acombination thereof, wherein the material is a block of material largeenough for the apparatus to be placed on. The conduit can be through anysuitable material. For example, the conduit can be through a materialthat includes copper, silicon carbide, graphite, cast iron, steel,stainless steel, a refractory material, a mixture of refractorymaterials, or a combination thereof. The cooling liquid can remove heatfrom the refractory material on which the bottom mold sits, therebyremoving heat from the bottom of the apparatus.

General

FIG. 4 illustrates a specific embodiment of an apparatus 400 fordirectional solidification of silicon, including a top heater partpositioned on top of a bottom mold 420. The top heater includes chains401 which are connected to the top heater 410 via holes 402 in verticalstructural members 403. The chains 401 form a bridle, which can allowthe top heater to be moved by the use of a crane. The apparatus can alsobe moved, for example, by placing the bottom half of the apparatus on ascissor lift while leaving the top heater over the bottom half. Theapparatus can be moved in any suitable fashion. The vertical structuralmembers 403 extend vertically from the bottom edge of the outer jacketof the top heater 410 to the top edge of the stainless steel outerjacket of the top heater 410. The vertical structural members arelocated on the outside of the top heater outer jacket and extend fromthe jacket parallel to a direction that is away from the center of thetop heater. The top heater also includes a horizontal structural member404, which is located on the outside of the top heater outer jacket andextends from the jacket in a direction that is parallel to a directionthat is away from the center of the top heater. The top heater alsoincludes a lip 405 that is part of the outer jacket of the top heater.The lip protrudes away from the outer jacket of the top heater. The lipcan extend inward toward the center axis of the top heater such that itcovers insulation of the top heater to any suitable extent.Alternatively, the lip can extend inward only enough to cover the bottomedge of the outer jacket of the top heater. Screen boxes 406 encloseends of the heating members that protrude from the outer jacket of thetop heater, protecting users from the heat and electricity that can bepresent in and near the ends of these members.

In the specific embodiment depicted in FIG. 4, insulation 411 from thebottom mold 420 is between the top heater 410 and the bottom mold 420.At least part of the one or more insulating layers of the bottom moldextends above the height of the outer jacket of the bottom mold. Thebottom mold includes vertical structural members 412. The verticalstructural members 412 are on the outer surface of the outer jacket ofthe bottom mold, extending away from the outer jacket parallel to adirection that is away from the center of the bottom mold. The verticalstructural members 412 extend vertically from the bottom edge of theouter jacket to the top edge of the outer jacket. The bottom mold alsoincludes horizontal structural members 413. The horizontal structuralmembers 413 are on the outer surface of the outer jacket of the bottommold, extending away from the outer jacket parallel to a direction thatis away from the center of the bottom mold. The horizontal structuralmembers 413 extend horizontally around the circumference of the bottommold. The bottom mold also includes bottom structural members 414 and415. The bottom structural members 414 and 415 extend away from theouter jacket parallel to a direction that is away from the center of thebottom mold. The bottom structural members extend across the bottom ofthe bottom mold. Some of the bottom structural members 415 are shapedsuch that they allow a forklift or other machine to lift or otherwisephysically manipulate the apparatus.

FIG. 5 illustrates a view towards the bottom of a top heater 500, partof an embodiment of an apparatus for directional solidification ofsilicon. In this specific embodiment, the outer jacket extends over partof the insulating layer 520 at the bottom edge of the top heater. Theheating members 530 are at equal heights, and the bottom edge of theouter jacket of the top heater 510 and the insulation 520 is below theheight of the heating elements.

FIG. 6 illustrates a view towards the inside of the directionalsolidification mold of an embodiment of an apparatus 600 for thedirectional solidification of silicon. In this specific embodiment, theedge of the outer jacket 610 does not extend over the edge of theinsulating layer 620. Rather, the insulating layer 620 extends over thetop edge of the directional solidification mold 630. The top edge of thedirectional solidification mold is below the height of the top edge ofthe insulation 620 and the outer jacket 610. The overallthree-dimensional shape of the embodiment is similar to that of a largethick-walled bowl.

FIG. 7 illustrates a silicon ingot 700 generated by embodiment of anapparatus and method of the present invention. The ingot is shown withthe bottom of the ingot 701 facing upwards, and the top of the ingot 702facing downwards. After being generated by directional crystallizationin an embodiment of the present invention, ingot 700 has the greatestamount of impurities in the last-to-freeze portion, at the top of theingot 702. Thus, in some embodiments, the top of the ingot 702 isremoved using, for example, a band saw, to increase the overall purityof the ingot 700.

Method of Using

The present invention provides a method of purifying silicon using theapparatus described above, where the apparatus can be any embodiment ofthe apparatus. The method of purifying silicon can be a method of makingone or more silicon ingots for cutting into one or more solar wafers.The method includes providing or receiving a first silicon. The firstsilicon can include silicon of any suitable grade of purity. The methodcan include at least partially melting the first silicon. The method caninclude fully melting the first silicon. At least partially melting thefirst silicon can include completely melting the first silicon, almostcompletely melting the first silicon (over about either 99%, 95%, 90%,85%, or 80% melted by weight), or partially melting the first silicon(less than about 80% or less melted by weight). Melting the firstsilicon provides a first molten silicon. The method includes providingor receiving a directional solidification apparatus. The directionalsolidification apparatus can substantially similar to that describedabove. The method includes directionally solidifying the first silicon,to provide a first molten silicon. In the directional solidification,the silicon solidifies approximately starting at the bottom of thedirectional solidification mold, and approximately ending at the top ofthe directional solidification mold. The directional solidificationprovides a second silicon. The last-to-freeze portion of the secondsilicon includes a greater concentration of impurities than the firstsilicon. The portions of the second silicon other than thelast-to-freeze portion include a lower concentration of impurities thanthe first silicon.

The second silicon can be a silicon ingot. The silicon ingot cansuitable for cutting into solar wafers, for the manufacture of solarcells. The silicon ingot can be cut into solar wafer using, for example,a band saw, a wire saw, or any suitable cutting device.

In some embodiments, the method is performed in a vacuum, in an inertatmosphere, or in ambient air. To perform the method in a vacuum or inan inert atmosphere, the apparatus can be placed in a chamber that iscapable of being made a less than atmospheric pressure or of beingfilling with an atmosphere with a greater concentration of inert gassesthan ambient air. In some embodiments, argon can be pumped into theapparatus or into a chamber holding the apparatus, to displace oxygenfrom the apparatus.

In some embodiments, the method includes positioning the top heaterdescribed above over the directional solidification mold. The bottommold, including the directional solidification mold, can be preheatedbefore molten silicon is added. The top heater can be used to preheatthe bottom mold. Preheating the bottom mold can help to preventexcessive quick solidification of silicon on the walls of the mold. Thetop heater can be used to melt the first silicon. The top heater can beused to transfer heat to the silicon, after it is melted. The top heatercan transfer heat to the silicon after it is melted when the silicon ismelted in the directional solidification mold. The top heater can beused to control the heat of the top of the silicon. The top heater canbe used as an insulator, to control the amount of heat loss at the topof the bottom mold. The first silicon can be melted outside theapparatus, such as in a furnace, and then added to the apparatus. Insome embodiments, silicon that is melted outside the apparatus can befurther heated to a desired temperature using the top heater after beingadded to the apparatus.

In embodiments including a top heater including an induction heater, thesilicon will preferably be melted prior to being added to the bottommold. Alternatively, the top heater will include heating elements aswell as induction heaters. Induction heating can be more effective withmolten silicon. Induction can cause mixing of the molten silicon. Insome embodiments, the power can adjusted sufficiently to optimize theamount of mixing, as too much mixing can improve segregation ofimpurities but can also create undesirable porosity in the final siliconingot.

The directional solidification can include the removal of heat from thebottom of the directional solidification apparatus. The removal of heatcan occur in any suitable fashion. For example, the removal of heat caninclude blowing fans across the bottom of the directional solidificationapparatus. The removal of heat can include allowing ambient air to coolthe bottom of the apparatus, without the use of fans. The removal ofheat can include running a cooling liquid through tubes adjacent to thebottom of the apparatus, though tubes that run through the bottom of theapparatus, through tubes that run through a material on which theapparatus sits, or a combination thereof. Removal of heat from thebottom of the apparatus allows a thermal gradient to be established inthe apparatus that causes directional solidification of the moltensilicon therein approximately from the bottom of the directionalsolidification mold to the top of the mold.

Removal of heat from the bottom of the apparatus can be performed forthe entire duration of the directional solidification. Multiple coolingmethods can be used. For example, the bottom of the apparatus can beliquid cooled and cooled with fans. Fan cooling can occur for part ofthe directional solidification, and liquid cooling for another, with anysuitable amount of overlap or lack thereof between the two coolingmethods. Cooling with liquid can occur for part of the directionalsolidification, and ambient air cooling alone for another part, with anysuitable amount of overlap or lack thereof between the two coolingmethods. Cooling by setting the apparatus on a cooled block of materialcan also occur for any suitable duration of the directionalsolidification, including in any suitable combination with other coolingmethods with any suitable amount of overlap. Cooling of the bottom canbe performed while heat is being added to the top; for example, whileheat is added to the top to increase the temperature of the top, tomaintain the temperature of the top, or to allow a particular rate ofcooling of the top. All suitable configurations and methods of heatingthe top of the apparatus, cooling the bottom, and combinations thereof,with any suitable amount of temporal overlap or lack thereof, areencompassed as embodiments of the present invention.

The directional solidification can include using the top heater to heatthe silicon to at least about 1450° C., and slowly cooling thetemperature of the top of the silicon from approximately 1450 to 1410°C. over approximately 10 to 16 hours. The directional solidification caninclude using the top heater to heat the silicon to at least about 1450°C., and holding the temperature of the top of the silicon approximatelyconstant at between approximately 1425 and 1460° C. for approximately 14hours. The directional solidification can include turning off the topheater, allowing the silicon to cool for approximately 4-12 hours, andthen removing the top heater from the directional solidification mold.

In one embodiment, the directional solidification includes using the topheater to heat the silicon to at least about 1450° C., and holding thetemperature of the top of the silicon approximately constant at betweenapproximately 1425 and 1460° C. for approximately 14 hours. Theembodiment includes turning off the top heater, allowing the silicon tocool for approximately 4-12 hours, and then removing the top heater fromthe directional solidification mold.

In another embodiment, the directional solidification includes using thetop heater to heat the silicon to at least about 1450° C., and slowlycooling the temperature of the top of the silicon from approximately1450 to 1410° C. over approximately 10 to 16 hours. The embodimentincludes turning off the top heater, allowing the silicon to cool forapproximately 4-12 hours, and then removing the top heater from thedirectional solidification mold.

The method can include removing the second silicon from the directionalsolidification apparatus. The silicon can be removed by any suitablemethod. For example, the silicon can be removed by inverting theapparatus and allowing the second silicon to drop out of the directionalsolidification mold. In another example, the directional solidificationapparatus is parted down the middle to form two halves, allowing thesecond silicon to be easily removed from the mold.

The method can include removing any suitable section from thedirectionally solidified second silicon. Preferably, the removal of thesuitable section leads to an increase in the overall purity of thesilicon ingot. For example, the method can include removing from thedirectionally solidified second silicon at least part of thelast-to-freeze section. Preferably, the last-to-freeze section of thedirectionally solidified silicon is the top of the ingot, as it isoriented during the bottom-to-top directional solidification. Thegreatest concentration of impurities generally occurs in thelast-to-freeze section of the solidified silicon. Removing thelast-to-freeze section thus can remove impurities from the solidifiedsilicon, resulting in a trimmed-second silicon with a lowerconcentration of impurities than the first silicon. The removal of asection of the silicon can include cutting the solid silicon with a bandsaw. The removal of a section of the silicon can include shot blastingor etching. Shot blasting or etching can also be used generally to cleanor remove any outer surface of the second silicon, not just thelast-to-freeze portion.

EMBODIMENTS

The present invention provides for the following exemplary embodiments:

Embodiment 1

An apparatus for directional solidification of silicon, comprising:

a directional solidification mold including at least one refractorymaterial;

an outer jacket; and

an insulating layer disposed at least partially between the directionalsolidification mold and the outer jacket.

Embodiment 2

The apparatus of embodiment 1, wherein the directional solidificationmold, the outer jacket and the insulating layer are configured to beused more than twice for the directional solidification of silicon.

Embodiment 3

The apparatus of any one of embodiments 1-2, wherein the directionalsolidification mold includes a capacity to receive at least about 1.4metric tonnes of silicon.

Embodiment 4

The apparatus of any one of embodiments 1-3, wherein the at least onerefractory material comprises aluminum oxide, silicon oxide, magnesiumoxide, calcium oxide, zirconium oxide, chromium oxide, silicon carbide,graphite, or a combination thereof.

Embodiment 5

The apparatus of any one of embodiments 1-4, wherein one or more sidewalls of the directional solidification mold include a differentthickness, material composition or amount of material relative to abottom of the directional solidification mold.

Embodiment 6

The apparatus of any one of embodiments 1-5, wherein one or more sidewalls of the directional solidification mold include a hot facerefractory, and wherein a bottom of the directional solidification moldincludes a conducting refractory.

Embodiment 7

The apparatus of any one of embodiments 1-6, wherein one or more sidewalls of the directional solidification mold include aluminum oxide, andwherein a bottom of the directional solidification mold includes siliconcarbide, graphite, or a combination thereof.

Embodiment 8

The apparatus of any one of embodiments 1-7, wherein the insulatinglayer includes an insulating brick, a refractory material, an insulatingboard, a ceramic paper, a high temperature wool, or a combinationthereof.

Embodiment 9

The apparatus of any one of embodiments 1-8, wherein one or more sidewalls of the insulating layer include a different thickness, compositionof material or amount of material relative to a bottom of the insulatinglayer.

Embodiment 10

The apparatus of any one of embodiments 1-9, wherein the insulatinglayer is disposed at least partially between one or more side walls ofthe directional solidification mold and one or more side walls of theouter jacket, and wherein the insulating layer is not disposed between abottom of the directional solidification mold and a bottom of the outerjacket.

Embodiment 11

The apparatus of any one of embodiments 1-10, wherein the outer jacketincludes steel, stainless steel, copper, cast iron, a refractorymaterial, a mixture of refractory materials, or a combination thereof.

Embodiment 12

The apparatus of any one of embodiments 1-11, further comprising a draftsufficient to allow the removal directionally solidified silicon fromthe mold.

Embodiment 13

The apparatus of any one of embodiments 1-12, wherein the outer jacketfurther comprises one or more structural members, the structural membersincluding steel, stainless steel, copper, cast iron, a refractorymaterial, a mixture of refractory materials, or a combination thereof.

Embodiment 14

The apparatus of any one of embodiments 1-13, further comprising atleast one liquid conduit, the liquid conduit sized and shaped to allow acooling liquid to pass therethrough and transfer heat away from thedirectional solidification mold.

Embodiment 15

The apparatus of embodiment 14, further comprising the cooling liquidselected from a group consisting of water, ethylene glycol, diethyleneglycol, propylene glycol, an oil, a mixture of oils, or a combinationthereof.

Embodiment 16

The apparatus of any one of embodiments 14-15, wherein the at least oneliquid conduit includes copper, cast iron, steel, stainless steel, arefractory material, a mixture of refractory materials, or a combinationthereof.

Embodiment 17

The apparatus of any one of embodiments 14-16, wherein the at least oneliquid conduit is located adjacent to or within a bottom portion of thedirectional solidification mold, the outer jacket or the insulatinglayer.

Embodiment 18

The apparatus of any one of embodiments 1-17, wherein the directionalsolidification mold comprises a top layer including at least oneslip-plane refractory material, the top layer configured to protect theremainder of the directional solidification mold from damage whendirectionally solidified silicon is removed therefrom.

Embodiment 19

The apparatus of embodiment 18, wherein the top layer includes asubstantially consistent thickness and composition throughout.

Embodiment 20

The apparatus of any one of embodiments 18-19, wherein the at least oneslip-plane refractory includes fused silicon dioxide, silicon dioxide,aluminum oxide, silicon nitride, graphite, or a combination thereof.

Embodiment 21

The apparatus of any one of embodiments 18-20, where the top layer isconfigured to be replaced or repaired between a first directionalsolidification of silicon and a second directional solidification ofsilicon.

Embodiment 22

The apparatus of any one of embodiments 1-21, further comprising a topheater including one or more heating members, the one or more heatingmembers including a heating element or an induction heater.

Embodiment 23

The apparatus of embodiment 22, wherein the heating elements includesilicon carbide, molybdenum disilicide, graphite, or a combinationthereof.

Embodiment 24

The apparatus of any one of embodiments 22-23, further comprising anelectrical system configured to supply power to the one or more heatingmembers, the electrical system coupled to the heating members such thatif any one heating element fails, any remaining heating elementscontinue to receive electricity and to produce heat.

Embodiment 25

The apparatus of any one of embodiments 22-24, wherein the top heaterfurther comprises insulation, the insulation including an insulatingbrick, a refractory, a mixture of refractories, an insulating board, aceramic paper, a high temperature wool, or a mixture thereof.

Embodiment 26

The apparatus of any one of embodiments 22-25, wherein the top heaterfurther comprises an outer jacket, wherein the insulation is disposed atleast partially between the one or more heating elements and the topheater outer jacket.

Embodiment 27

The apparatus of embodiment 26, wherein the top heater outer jacketfurther comprises one or more structural members, the structural membersincluding steel, stainless steel, copper, cast iron, a refractorymaterial, a mixture of refractory materials, or a combination thereof.

Embodiment 28

The apparatus of any one of embodiments 26-27, wherein the top heaterouter jacket includes stainless steel.

Embodiment 29

The apparatus of any one of embodiments 1-28, configured such that afirst half is separable from a second half, along a vertical seam,sufficient to remove directionally solidified silicon, and

wherein the first half and the second half can be joined to form theapparatus, and the apparatus can be reused.

Embodiment 30

A method of purifying silicon, comprising:

providing or receiving a first silicon;

at least partially melting the first silicon, to provide a first moltensilicon; and

directionally solidifying the first molten silicon in the directionalsolidification apparatus of any one of embodiments 1-29, to provide asecond silicon.

Embodiment 31

A method of purifying silicon, comprising:

providing or receiving a first silicon; providing or receiving adirectional solidification apparatus, wherein the apparatus comprises:

-   -   a directional solidification mold including at least one        refractory material;    -   an outer jacket; and    -   an insulating layer disposed at least partially between the        directional solidification mold and the outer jacket;

at least partially melting the first silicon to provide a first moltensilicon; and

directionally solidifying the first molten silicon in the directionalsolidification mold to provide a second silicon.

Embodiment 32

The method of embodiment 31, wherein the method of purifying silicon canbe a method of making one or more silicon ingots for cutting into one ormore solar wafers.

Embodiment 33

The method of any one of embodiments 31-32, further comprisingpositioning a heater over the directional solidification mold, includingpositioning one or more heating members selected from a heating elementand an induction heater over the directional solidification mold.

Embodiment 34

The method of any one of embodiments 31-33, further comprising addingthe first molten silicon to the directional solidification apparatus,

wherein melting the first silicon includes melting the first siliconoutside the directional solidification apparatus.

Embodiment 35

The method of any one of embodiments 31-34, wherein melting the firstsilicon including melting the first silicon inside the directionalsolidification apparatus prior to the directional solidification.

Embodiment 36

The method of any one of embodiments 31-35, wherein directionallysolidifying the first molten silicon comprises removing heat from abottom of the directional solidification apparatus.

Embodiment 37

The method of any one of embodiments 31-36, further comprising removingthe second silicon from the directional solidification apparatus.

Embodiment 38

The method of any one of embodiments 33-37, wherein the heating elementinclude silicon carbide, molybdenum disilicide, graphite, or acombination thereof.

Embodiment 39

The method of any one of embodiments 33-38, wherein directionallysolidifying the first molten silicon comprises

using the top heater to heat the silicon to at least about 1450° C.; and

slowly cooling the temperature of a top portion of the silicon fromapproximately 1450° C. to 1410° C. over approximately 10 to 16 hours.

Embodiment 40

The method of any one of embodiments 33-39, wherein directionallysolidifying the first molten silicon comprises:

using the top heater to heat the silicon to at least about 1450° C.;and,

holding the temperature of the top of the silicon approximately constantat between approximately 1425° C. and 1460° C. for approximately 14hours.

Embodiment 41

The method of any one of embodiments 33-40, wherein directionallysolidifying the first molten silicon comprises

turning off the top heater;

allowing the silicon to cool for approximately 4-12 hours; and

removing the top heater from the directional solidification mold.

Embodiment 42

The method of any one of embodiments 36-41, wherein removing heat fromthe bottom of the directional solidification apparatus includes coolingthe bottom of the apparatus with one or more fans.

Embodiment 43

The method of any one of embodiments 36-42, wherein removing heat from abottom of the directional solidification apparatus includes cooling thebottom of the apparatus with a cooling liquid.

Embodiment 44

The method of any one of embodiments 37-43, further comprising removingfrom the directionally solidified second silicon at least part of thelast-to-freeze section.

Embodiment 45

An apparatus for directional solidification of silicon, comprising:

a directional solidification mold including a refractory material,

-   -   wherein one or more side walls of the directional solidification        mold include aluminum oxide;    -   wherein a bottom of the directional solidification mold includes        silicon carbide, graphite, or a combination thereof;    -   a top layer, including a slip-plane refractory, the top layer        configured to protect the remainder of the directional        solidification mold from damage when directionally solidified        silicon is removed from the mold;

an outer jacket, including steel;

an insulating layer, including insulating brick, a refractory material,a mixture of refractory materials, insulating board, ceramic paper, hightemperature wool, or a mixture thereof, the insulating layer disposed atleast partially between one or more side walls of the directionalsolidification mold and one or more side walls of the outer jacket;

a top heater, comprising:

-   -   one or more heating members, each of the heating members        including a heating element or an induction heater;        -   wherein the heating element includes silicon carbide,            molybdenum disilicide, graphite, or a combination thereof;    -   insulation, including insulating brick, a refractory, a mixture        of refractories, insulating board, ceramic paper, high        temperature wool, or a combination thereof;    -   an outer jacket, including stainless steel;    -   wherein the insulation is disposed at least partially between        the one or more heating members and the top heater outer jacket;

wherein the directional solidification mold, the outer jacket and theinsulating layer are configured to be used more than twice for thedirectional solidification of silicon.

EXAMPLES Example 1

Silicon carbide resistance elements were used in a top heater insulatedwith high temperature wool insulation and a steel shell. Molten silicon(1.4 tonnes) was poured into a refractory-lined preheated bottom sectionof the apparatus. The apparatus had aluminum oxide refractory wallsincluding a draft to allow the silicon to be dumped out after cooling.The walls of the refractory were coated with a thin slip-plane ofaluminum oxide refractory and then a second layer of Si₃N₄ powder. Thebottom of the directional solidification mold was made from siliconcarbide refractory and the outside of the steel shell was cooled withfans blowing air on the bottom of the outer shell. The heaters were setat 1450° C. for 14 hrs and then the elements were turned off. Six hourslater the top heater section was removed and the silicon was allowed tocool to room temperature. The mold was flipped over. The 1.4 tonne ingotwas cut in half and the top 25% of the ingot was cut off to removeimpurities. The grains were about 1-2 cm in width and 3-10 cm in height,forming columns in the vertical direction similar to a standard ingotfrom the Bridgeman process.

Example 2

Silicon carbide resistance elements were used in a top heater insulatedwith high temperature wool insulation and a steel shell. Molten silicon(0.7 tonnes) was poured into a refractory-lined preheated bottom sectionof the apparatus. The apparatus had aluminum oxide refractory wallsincluding a middle parting line to remove the silicon ingot. The wallsof the refractory were coated with a thin slip-plane of SiO₂ refractory.The bottom of the directional solidification mold was made from graphiteand the outside of the steel shell was cooled with fans blowing air onthe bottom of the outer shell. The heaters were set at 1450° C. for 12hours and then the elements were turned off. Six hours later the topheater section was removed and the silicon was allowed to cool to roomtemperature. The mold was opened at the parting line. The 0.7 tonneingot was cut in half and the top 15% of the ingot was cut off to removeimpurities. The grains were about 1 cm in width and 3-10 cm in height,forming columns in the vertical direction similar to a standard ingotfrom the Bridgeman process.

All publications, patents, and patent applications are incorporatedherein by reference. While in the foregoing specification this disclosedsubject matter has been described in relation to certain preferredembodiments thereof, and many details have been set forth for purposesof illustration, it will be apparent to those skilled in the art thatthe disclosed subject matter is susceptible to additional embodimentsand that certain of the details described herein can be variedconsiderably without departing from the basic principles of thedisclosed subject matter.

1. (canceled)
 2. An apparatus for directional solidification of silicon,comprising: a directional solidification mold having one or more sidewalls and a bottom portion, the one or more side walls including a firstmaterial, and the bottom portion including a second material differentfrom the first material; an outer jacket; and an insulating layerdisposed at least partially between the directional solidification moldand the outer jacket, the insulating layer in contact with at least aportion of the directional solidification mold, at least a portion of aninner side surface of the outer jacket, and at least a portion of aninner bottom surface of the outer jacket, and wherein the bottom portionof the direction solidification mold is in contact with the inner bottomsurface of the outer jacket.
 3. The apparatus of claim 2, wherein theinsulating layer is not disposed between the bottom portion of thedirectional solidification mold and the inner bottom surface of theouter jacket.
 4. The apparatus of claim 2, wherein the first materialincludes a hot face refractory.
 5. The apparatus of claim 2, wherein thefirst material includes aluminum oxide.
 6. The apparatus of claim 2,wherein the second material includes a conducting refractory.
 7. Theapparatus of claim 2, wherein the second material includes at least oneof silicon carbide, graphite, copper, steel, stainless steel, and castiron.
 8. The apparatus of claim 2, comprising a top layer in contactwith at least a portion of a top surface of the directionalsolidification mold, the top layer including a slip-plane refractory. 9.The apparatus of claim 8, wherein the top layer includes at least one offused silicon dioxide, silicon dioxide, aluminum oxide, silicon nitride,and graphite.
 10. The apparatus of claim 2, wherein the bottom portionincludes at least one appendage member along a side surface of thebottom portion.
 11. The apparatus of claim 10, wherein the one or moreside walls includes at least one receiving slot configured to receivethe at least one appendage member to secure the one or more side wallsto the bottom portion.
 12. The apparatus of claim 2, wherein theinsulating layer includes at least one of insulating brick, a refractorymaterial, a mixture of refractory materials, insulating board, ceramicpaper, and high temperature wool.
 13. The apparatus of claim 2, whereinthe insulating layer comprises an inner layer and an outer layer. 14.The apparatus of claim 13, wherein the inner layer includes a thirdmaterial and the outer layer includes a fourth material different fromthe third material.
 15. The apparatus of claim 2, comprising one or moreanchors extending through at least one of the insulating layer and theone or more side walls.
 16. The apparatus of claim 15, wherein the oneor more anchors include at least one of steel, stainless steel, and castiron.
 17. An apparatus for directional solidification of silicon,comprising: a directional solidification mold having one or more sidewalls and a bottom portion, the one or more side walls including a firstmaterial, and the bottom portion including a second material differentfrom the first material; an outer jacket; an insulating layer disposedat least partially between the directional solidification mold and theouter jacket, the insulating layer in contact with at least a portion ofthe directional solidification mold, a portion of an inner side surfaceof the outer jacket, and at least a portion of an inner bottom surfaceof the outer jacket, and wherein the bottom portion of the directionsolidification mold is in contact with the inner bottom surface of theouter jacket; and a top heater, comprising: one or more heating members;insulation; and a top heater outer jacket.
 18. The apparatus of claim17, wherein each of the heating members includes one of a heatingelement or an induction heater.
 19. The apparatus of claim 18, whereinthe heating element includes at least one of silicon carbide, molybdenumdisilicide, and graphite.
 20. The apparatus of claim 17, wherein theinsulation includes at least one of insulating brick, a refractorymaterial, a mixture of refractory materials, insulating board, ceramicpaper, and high temperature wool.
 21. The apparatus of claim 20, whereinthe top heater outer jacket includes stainless steel.